Time-of-the-day not set-please run SETUP Program

I have a three-year old Optiplex 5060 desktop. When I turn it on, I get the message ''Time-of-the-day not set - please run SETUP program. Invalid configuration information - please run SETUP program. Press F1 key to retry boot. Press F2 key for setup utility. Press F5 key to run onboard diagnostics''. I press F1, and the desktop starts working properly. The desktop is connected to a ups. When it is connected to the ups, it seems to have more difficulty starting.  It is as if there is an instant voltage drop and then the system regains its balance. Once it did not even start when it was connected to the ups. However, whether it is connected to the ups or not, there is always the above-mentioned message. The desktop has passed DELL's diagnostics tests successfully. The diagnostic test checked the battery too. Nothing faulty was detected.